Stimulation of IgE receptors on rat mast cells and basophilic leukemia cells causes a transient increase of phospholipid methylation followed by disappearance of methylated lipids due to the activation of phospholipase A2. In rat mast cells, blocking phospholipase A2 results in the inhibition of Ca influx as well as in the inhibition of arachidonic acid release. The close relation between phospholipid methylation and Ca2 ions influx is found in rabbit peritoneal leucocytes, human fibroblast, and mouse lymphocytes. Phospholipiase A2 is regulated by its specific inhibitory protein induced by corticoid in rabbit neutrophils and human fibroblast. The phospholipase A2 inhibitory protein is being isolated and characterized.